Composite metal-coated polyester films with barrier properties

ABSTRACT

The invention concerns composite metallized polyester films with good barrier properties relative to gases. More precisely, it concerns a biaxially drawn composite metallized film based on polyester and having improved barrier properties to gases, characterized in that the composite comprises a polyester film base of 5 μm to 50 μm, directly coated on one of its two faces with a layer including a mixture of a polyvinyl alcohol which has an average degree of polymerization in number equal to or in excess of 350 and a water-dispersible copolyester with sulphonyloxy residues, the layer including the polyvinyl alcohol and the water-dispersible copolyester being in turn coated with a metal layer, the composite film exhibiting a permeability to oxygen, measured at 23° C. under 50% relative humidity, equal to or lesser than 0.4 cm 3 /m 2 /24 h. The composite metallized polyester films have, besides their barrier properties, excellent resistance to mechanical stresses, such as in particular crumpling.

The present invention is concerned with metallized composite films basedon polyester and having good barrier properties against gases.

Polyester films, in particular polyethylene terephthalate, are largelyused for packaging, owing to their numerous advantages, such as theirmechanical properties, their transparency, their lack of toxicity, andbecause they have no taste or odour.

However, their properties as barriers for gases can limit their use inapplications requiring a high level of protection for the packageditems, against the action of outside gases, in particular against theaction of the oxygen of air, or conversely, can not maintain a constantgas composition within the packaging.

In order to overcome this drawback, it was proposed in the GB patent 1126 952 to deposit on a polymer film a solution of a polyvinyl alcohol,in order to create a layer of polyvinyl alcohol, the latter having goodgas-barrier properties. In order to enable a good adhesion of thepolyvinyl alcohol layer on films of acetate cellulose, polycarbonate orpolyethylene terephthalate, an intermediate layer of a polyurethaneadhesive is provided between the film base and the polyvinyl alcohollayer.

Patent EP-A-0 254 468 also describes a composite film comprising a filmbase of a synthetic thermoplastic polymer, such as a polyamide, apolyethylene, a polypropylene or a polyester, having two coatings on thesame face of the film base; the first coating being adjacent to the filmbase and being comprised of a urethane primer applied in a solvent,which enables, when dry, a dispersion in an aqueous solution of apolyvinyl alcohol to wet the primer coating, the second coating beingplaced on the dried surface of the first coating and including amaterial based on a polyvinyl alcohol acting as a barrier to gases.

The present invention is concerned with metallized polyester filmshaving good barrier properties to gases, owing to a coating including apolyvinyl alcohol placed directly on the surface of the polyester filmwithout the presence of an intermediate adhesive layer, while providinggood adhesion properties between the polyester film base and thecoating.

More precisely, it is concerned with a metallized composite filmstretched biaxially, based on a polyester and having improved barrierproperties to gases, characterised in that it includes a polyester filmbase of 5 μm to 50 μm, coated on one of its two faces with a layerincluding a polyvinyl alcohol which has an average degree ofpolymerization in number equal to or in excess of 350 and awater-dispersible copolyester with sulphonyloxy residues, the layerincluding the polyvinyl alcohol and the water-dispersible copolyesterbeing in turn coated with a metal layer, said composite film exhibitinga permeability to oxygen measured at 23° C. under a relative humidity of50%, equal to or lesser than 0.4 cm³/m²/24 h.

The polyester forming the film base can be selected from thosepolyesters which are normally used for obtaining semi-crystallinebi-oriented films. These are film-forming linear polyesters, whichcrystallise when oriented and which are obtained in the usual mannerfrom one or several aromatic dicarboxylic acids or their derivatives(for example, esters of lower aliphatic alcohols or halogenides) andfrom one or several aliphatic glycols. As examples of aromatic acids,one can mention phthalic acid, terephthalic acid, isophthalic acid,naphthalene 2,5-dicarboxylic acid and naphthalene 2,6-dicarboxylic acid.These acids can be associated to a minor amount of one or severalaliphatic or cycloaliphatic dicarboxylic acids, such as adipic acid,azelaic acid and hexahydroterephthalic acid. As non limiting examples ofaliphatic diols, one can mention ethylene glycol, 1,3-propanediol and1,4-butanediol. These diols can be associated with a minor amount of oneor several aliphatic diols with a higher carbon content (neopentylglycolfor example) or cycloaliphatic diols (cyclohexanedimethanol).Preferably, the film-forming crystallisable polyesters arepolyterephthalates or polynaphthalene dicarboxylates of alkylene diolsand, in particular polyterephthalate of ethylene glycol (PET), of1,4-butanediol or copolyesters including at least 80% in moles ethyleneglycol terephthalate residues. Advantageously, the polyester is apolyterephthalate of ethylene glycol having an intrinsic viscositymeasured at 25° C. in ortho-chlorophenol comprised between 0.6 and 0.75dl/g.

The polyester forming the film base must be selected in such a mannerthat its temperature at the beginning of the melting be higher than thetemperature at which the bi-stretched film comprising the layer of thepolyvinyl alcohol and of the water-dispersible copolyester is heatedduring its preparation.

When the thickness of the film including the polyvinyl alcohol and awater-soluble copolyester with sulphonyloxy residues is lesser than 0.6μm, the average roughness Rz of the film base (such as defined instandard DIN 4768) is lesser than or equal to 0.40 μm on the face of thefilm carrying the layer of the polyvinyl alcohol and of thewater-dispersible copolyester and this face exhibits, on the average,not more than 20 peaks having a height equal to or in excess of 1micrometre and not more than 150 peaks having a height comprised between0.4 and 1 micrometre, per square millimetre.

The peak height distribution mentioned above for defining the surfacetopography of the polyester film base, can be determined, in a knownmanner, and in particular through visual inspection with aninterferential microscope which makes it possible to count the number ofinterference rings for a light having a known wavelength. Theinterferometers most frequently used are the NOMARSKI, the MIRAU and theMICHELSON interferometers.

Concerning the industrial implementation of these processes for thepreparation of films according to the invention, in which the speed ofthe machine in which the films are processed is generally in excess of100 meters per minute, it is particularly preferable that the face ofthe film base carrying the layer of polyvinyl alcohol and of thewater-dispersible copolyester have no more than an average of 20 peakshaving a height equal to or in excess of 1 micrometre and no more than100 peaks having a height comprised between 0.4 and 1 micrometre, persquare millimetre.

The polyvinyl alcohol entering into the composition of the layer of thepolyvinyl alcohol and of the water-dispersible copolyester can be anypolyvinyl alcohol. In practice, the polyvinyl alcohols generallyavailable commercially have a degree of hydrolysis of about 73% to morethan 99% (which means that they have a content of from about 73% to morethan 99% of vinyl alcohol residues in their formula). Except when statedotherwise, this does not exclude the use of polyvinyl alcohols with adegree of hydrolysis lesser than 73%, when available. In the presentdescription the terms <<degree of hydrolysis>>, <<degree ofsaponification>> and <<content in vinyl alcohol residues>> areinterchangeable.

The water-dispersible copolyesters with sulphonyloxy residues combinedwith the polyvinyl alcohol in the coating are copolyesters derived fromat least one aromatic dicarboxylic acid and from at least one aliphaticdiol, and carrying a plurality of sulphonyloxy groups of the generalformula (I):

—(—SO₃—)_(n)M  (I)

in which n is equal to 1 or 2, M is a hydrogen atom, an alkali metal, anearth-alkali metal, a cationic ammonium or a cationic quaternaryammonium.

The term <<water-dispersible copolyester>> designates in the presentapplication copolyesters which are soluble in water or copolyestersforming stable homogeneous dispersions.

The water-dispersible copolyesters carrying sulphonyloxy residues arealready known, in particular from patent FR 1 602 002 and from patentEP-A-0 540 374, to which one can refer for more detailed information ontheir preparation and their composition. They are obtained through thepolycondensation of one or of several aromatic dicarboxylic acids withone or several aliphatic diols and at least one difunctional compoundincluding at least one sulphonyloxy group of formula (I). Forconvenience sake, in the remainder of the specification, the term<<sulphonyloxy group>> designates both hydroxysulphonyl groups andalkali metal salts, alkaline-earth metal salts or ammonium salts derivedtherefrom.

Amongst the aromatic dicarboxylic acids used for preparing thewater-dispersible copolyesters, one can mention, by way of example,terephthalic acid, isophthalic acid, orthophthalic acid and naphthalene1,4-dicarboxylic acid. These acids can be used alone or as mixtures.Amongst the above-mentioned acids, one can use preferably, for thepreparation of water-dispersible copolyesters entering into thecomposition of the films of the invention, terephthalic acid orisophthalic acid alone, mixtures thereof, or their mixtures with otheraromatic dicarboxylic acids. Mixtures of terephthalic acid with one orseveral other aromatic dicarboxylic acids and specially with isophthalicacid, are particularly well-suited. In this case, the amount ofterephthalic acid, expressed in moles, can vary between 20 and 99% ofthe total number of moles of non sulphonated diacids and, preferably,between 30 and 95%.

Concerning the preparation of the water-dispersible copolyester,aliphatic dicarboxylic acids including 3 to 15 atoms of carbon can beassociated with the aromatic diacids. More particularly, a portion orthe totality of the aromatic dicarboxylic acid used with theterephthalic acid, such as isophthalic acid, can be replaced by one orseveral aliphatic acids such as adipic acid, glutaric acid, succinicacid, suberic acid, sebacic acid and dodecanoic acid.

As examples of diols entering into the composition of thewater-dispersible copolyesters with sulphonyloxy groups, one can mentionethylene glycol, 1,4-butanediol, 1,5-butanediol, 1,3-propanediol,1,2-propanediol, 2,2-dimethyl-1,3-propanediol (or neopentylglycol),1,5-pentanediol, 1,6-hexanediol, diethylene glycol, triethylene glycol,tetraethylene glycol and cyclohexane dimethanol. Ethylene glycol and itsoligomers are particularly suitable. They can be used either alone or inmixtures thereof and/or with other diols. Preferably, residues derivedfrom ethylene glycol and from its oligomers are present in thewater-dispersible copolyesters.

The sulphonyloxy groups of the water-dispersible copolyesters areintroduced via a difunctional compound carrying a sulphonyloxy group,capable of reacting with diols during the polycondensation. Examples ofsuch compounds are cited in the patent EP-A 0 540 374. Preferably, thesulphonyloxy residues are derived from salts of alkali metals ofaromatic dicarboxylic acids such as sulphoterephthalic acid,sulfoisophthalic acid, sulphophthalic acid, 4-sulphonaphthalene2,7-dicarboxylic acid,sulpho-4,4′-bis(hydroxycarbonyl)diphenylsulphones,sulphodiphenyidicarboxylic acid,sulpho4,4′-bis(hydroxycarbonyl)diphenylmethane,5-sulphophenoxyisophthalic acid.

Water-dispersible copolyesters including sodium5-oxysulphonylisophtalate residues are preferred.

In the water-dispersible copolyesters, the dicarboxylic acid residueswith sulphonyloxy groups represent 5-30 moles per 100 moles of thedicarboxylic acid residues and, preferably, 8-15 moles per 100 moles.

The coating of the polyvinyl alcohol and of the water-dispersiblecopolyester exhibits a weight ratio of the polyvinyl alcohol to thewater-dispersible copolyester of 99/1 to 50/50. Preferably, this weightratio of the polyvinyl alcohol to the water-dispersible copolyester iscomprised between 97/3 and 80 120 and, even more preferably between 95/5and 85/15.

The other face of the composite films according to the invention, theback face thereof, must exhibit slippage characteristics sufficient forenabling an easy handling of the film, in particular when stretching thesame on guide rollers and when winding up of the same on uptake rollers.

These slippage characteristics can be brought about in differentmanners. One of the most usual methods consists in incorporating intothe polyester, before its transformation into a film, solid inertfillers. These fillers are generally mineral fillers, such as forexample silica, titanium oxide, zirconium oxide, alumina, mixtures ofsilica and alumina, silicates, calcium carbonate and barium sulphate.These fillers can also be comprised of particles of polymers.

The average diameter in volume of the fillers is generally comprisedbetween 1 and 10 micrometres and, preferably, between 1 and 5micrometres.

The filler content of the film is usually comprised between 0.02% and 1%weight relative to the weight of the polyester.

An interesting version of the invention consists in having a polyesterfilm base having surface properties, in particular roughness properties,which are different for each one of its two faces. These differingaverage roughnesses Rz can, for example, be higher than 0.15 micrometreson the back face of the film and be equal to or lesser than 0.40micrometres on the face of the film base carrying the coating of thepolyvinyl alcohol and of the water-dispersible copolyester.

Thus, the polyester film base can be formed of two layers havingdiffering surface properties, in particular differing roughnessproperties.

Such dissymetrical films can be obtained using a co-extrusion techniqueof 2 polyesters having differing filler contents and, possibly fillersof differing nature. Advantageously, the polyester used is the same forthe two co-extruded layers and the layer which is to receive the coatingof polyvinyl alcohol and of the water-dispersible copolyester has alower filler content. The relative thickness of the 2 polyester layersforming the polyester film base can vary broadly.

Generally, the layer which has the lower filler content and receivingthe coating of the polyvinyl alcohol and of the water-dispersiblecopolyester has a thickness equal to or in excess of 0.5 μm and,preferably equal to or in excess of 1.0 μm.

Film bases of polyester exhibiting differing surface properties on eachone of the faces and obtained by other means known from prior art arenor excluded from the field of the invention.

One can thus confer, according to patent EP-A-0 378 954 good slippagecharacteristics to the back face of a film base by depositing on saidback face a modified polymer obtained through a radical polymerisationin an aqueous phase of at least one acrylic monomer and of a polyesterdispersible in water, derived from at least one aromatic dicarboxylicacid and from at least one aliphatic diol and including a plurality ofsulphonyloxy groups, in particular of sodium sulphonate groups.

Depending on the coating process used, the coating of the polyvinylalcohol and of the water-dispersible copolyester has a thickness equalto or lesser than 3 μm. This thickness can be, if desired, equal to orlesser than 0.60 μm or even lesser than 0.20 μm, should one wish tofacilitate the recycling of the film according to the invention. Inpractice however, only on rare occasions will the thickness be lesserthan 0.05 μm.

An advantageous version of the composite films of the invention isconcerned with films which carry a layer of polyvinyl alcohol on top ofthe coating of the polyvinyl alcohol and of the water-dispersiblecopolyester. This additional layer makes it possible to further decreasethe permeability to gases of the composite films. By way of example, itcan generally have a thickness of 0.1 μm to 10 μm.

The films of the invention carry a metal layer on the free face of thecoating of the polyvinyl alcohol and of the water-dispersiblecopolyester or, on the free face of the additional layer of polyvinylalcohol when such a layer is provided.

The metal layer which is deposited on the surface of the coating of thepolyvinyl alcohol and of the water-dispersible polyester which is not incontact with the polyester film or which is deposited on the free faceof the additional layer of the polyvinyl alcohol is made of a metalwhich can be deposited using different known techniques. The metal isselected most often from aluminium, copper, chromium, nickel and silver.For practical and economical reasons, the metal used is preferablyaluminium.

The thickness of the metal layer is generally between 0.01 μm and 0.06μm. This thickness is determined by the measure of the optical density(measurements carried out with an apparatus MACBETH-TD102). Thesethicknesses correspond to optical densities of 1 to 3.6.

On the free face of the polyester film base, one can also apply aheat-sealable layer which can be of the polyolefin type (such as:radical polyethylenes, linear polyethylenes, metallocene polyethylenes,polypropylenes, ethylene—vinyl acetate copolymers, ionomer resins)and/or copolyesters. These complex films can be obtained by laminationusing a single or a two component adhesive or by extrusion coating, byco-extrusion or by spread coating. This heat-sealable layer can functiononly as a sealable layer or have the double function of a sealable andof a peelable layer.

The complex films thus construed can be used for making packaging items,such as bags, membrane seals for trays, and secondary wrapping designedfor packaging products sensitive to oxidation. They are particularlywell adapted to packaging under a modified atmosphere.

The present invention is also concerned with a process for theproduction of the above-described filmy.

More particularly, it concerns a process for the production ofmetallized composite films based on a polyester film which includes onone of its faces a coating of a polyvinyl alcohol and of awater-dispersible polyester, possibly an additional layer of polyvinylalcohol, and a metal layer, said process being characterised in that thepolyester film base is coated on one of its faces with an aqueoussolution including a polyvinyl alcohol having, in a 4% aqueous solutionand at 20° C., a viscosity which is equal to or in excess of 4 mPa.s anda water-dispersible copolyester, the coating of the polyvinyl alcoholand of the water-dispersible polyester being in turn coated, if desired,with an aqueous solution of a polyvinyl alcohol, and in that said coatedfilm is subjected to a thermal treatment at a temperature equal to or inexcess of 170° C. and in that a metal layer is deposited on the coatingof the polyvinyl alcohol and of the water-soluble copolyester or, on theadditional layer of polyvinyl alcohol when such a layer is provided,

The thickness of the coating of the polyvinyl alcohol and of thewater-dispersible polyester is preferably equal to or lesser than 3micrometres.

The thickness of the additional layer of polyvinyl alcohol is,preferably, from 0.1 to 10 micrometres.

When it is desired to produce a coating of a polyvinyl alcohol and of awater-dispersible ester which has a thickness lesser than 0.6micrometres, it is preferable that the face of the polyester film onwhich is applied the solution of the polyvinyl alcohol and of thewater-dispersible copolyester to produce a coating exhibits an averagesurface roughness Rz lesser than or equal to 0.40 μm and includes on theaverage not more than 20 peaks having a height equal to or in excess of1 micro-metre and not more than 150 peaks having a height comprisedbetween 0.4 and 1 micrometre per square millimetre.

The coating of the polyester film base by the aqueous solution of thepolyvinyl alcohol and of the water-dispersible copolyester can becarried out on-line or as a separate operation. When the coatingoperation is carried out on-line, the thickness of the coating is lesserand it is preferable to have a film surface topography which is morestrictly defined.

In such a case, when the process according to the invention isimplemented, for example in an industrial production using a high-speedequipment (generally running at more than 100 metres per minute), it isadvantageous that the coating by the aqueous solution of the polyvinylalcohol and of the water-dispersible copolyester be carried out on aface of the polyester film carrying, on the average, not more than 20peaks having a height equal to or in excess of 1 micrometre and not morethan 100 peaks having a height comprised between 0.4 and 1 micrometre,per square millimetre.

When one carries out a coating operation as a separate step, thepolyester film surface can be rougher than in the case of the on-linecoating, without this however being a necessity.

Before the coating operation of the film base, the surface thereof isgenerally subjected to a physical treatment (such as a corona, flame orplasma treatment) designed for ensuring a good spreading of thepolyvinyl alcohol layer on said film base. This treatment makes itpossible to achieve a value for the surface tension of the filmstretched monoaxially which is in excess of that of the coating of thepolyvinyl alcohol and of the water-dispersible copolyester and which ispreferably equal to or in excess of 54 mN.m.

The aqueous solution of the polyvinyl alcohol and of thewater-dispersible copolyester used has generally a concentration of 1%to 20% in weight/weight and, preferably of 5% to 15% in weight/weight.Generally, in the first instance, a solution of polyvinyl alcohol isprepared. This solution is prepared carefully first at room temperatureby simply stirring, then by heating to a temperature not exceeding 95°C.; after cooling, the solution is filtered. This solution must begel-free. The absence of gel must be controlled by turbidity, dry mattercontent and refractive index measurements. The water-dispersiblecopolyester is dissolved or dispersed in this solution of polyvinylalcohol.

The weight ratio of the polyvinyl alcohol to the water dispersiblecopolyester in said aqueous solution is from 99/1 to 50/50 and rangespreferably from 97/3 to 80/20 and even more preferably from 95/5 to85/15.

The polyvinyl alcohol used in the process according to the invention isa compound available commercially. It can be used as such or prepared inparticular by hydrolysis of vinyl carboxylates, more particularly vinylpolyacetates or their copolymers rich in vinyl acetate residues, such asthe copolymers of vinyl acetate and ethylene (or EVA). As was mentionedpreviously, the polyvinyl alcohol used includes a percentage of vinylalcohol residues which is not critical. By way of example, one canhowever indicate a degree of hydrolysis of the polyvinyl alcoholgenerally from 73 to 99%.

The polyvinyl alcohol used for the preparation of the composite coatingof the polyvinyl alcohol and of the water-dispersible copolyester of thecomposite film of the invention exhibits a viscosity in a 4% aqueoussolution and at 20° C., measured in an apparatus of the Brookfield type,which is equal to or in excess of 4 centipoises (or 4 mPa.s), whichcorresponds, within experimental errors, to an average polymerisationdegree in number equal to or in excess of 350.

The choice of the concentration of the solution of the polyvinyl alcoholand of the water-dispersible copolyester, as well as the apparatus usedfor the coating, is in particular, dependent on the desired thicknessfor the final coating of the polyvinyl alcohol and of thewater-dispersible copolyester.

Coating operations are usually carried out with gravure rollersaccording to the reverse gravure technique. However, the process of theinvention is not limited to this embodiment.

In the case of an on-line coating, the polyester film base is, the moreoften, stretched longitudinally (i.e. in the machine direction) beforesaid coating by means of an aqueous solution of the polyvinyl alcoholand of the water-dispersible copolymer, without this version limitingthe invention.

This stretching can be carried out in one or in several steps, which isalso the case of the stretching which follows the coating.

The temperature at which the coated film is treated is preferablybetween 180° C. and 240° C. or, even more preferably, between 200° C.and 230° C.

The polyester film base can be prepared by extrusion of a polyester suchas defined above, including one or several inert fillers to provide, inparticular, sufficient slippage characteristics. It can also be preparedby co-extrusion, on the one hand of a polyester low in filler and, onthe other hand, of a filled polyester.

In a second version, the polyester film base comprises a filled roughlayer providing said back face and a less filled layer, providing saidfront face, which will be coated with the solution of the polyvinylalcohol and of the water-dispersible copolyester.

The filled polyester layer generally exhibits an average roughness Rzwhich is equal to or in excess of 0.15 micrometres and, preferably equalto or in excess of 0.30 micrometres, whereas the less filled or nonfilled front layer exhibits a total roughness equal to or lesser than0.40 micrometres and preferably, equal to or lesser than 0.25micrometres; as indicated previously, the face of the film base, whichwill be coated with the solution of the polyvinyl alcohol an of thewater-dispersible copolyester, carries preferably, on the average, notmore than 20 peaks having a height equal to or in excess of 1 micrometreand not more than 100 peaks having a height comprised between 0.4 and 1micrometre per square millimetre.

The deposition of the metal coating is carried out by any known means.Preferably, use is made of a vacuum metallization technique.

The composite films of the invention exhibit excellent properties ofresistance against mechanical aggression, such as in particular,crumpling or folding. This means that the barrier properties alreadydiscussed are little or not diminished after such a mechanicalaggression.

The following examples illustrate the invention

EXAMPLES GENERAL EXPERIMENTAL PROCEDURE

A composite film is prepared from polyethylene terephthalate (PET)through the co-extrusion on the one hand of a PET containing 0.7% silicaobtained by precipitation of gels and exhibiting an average diameter of3.3 micrometres (size measurements carried out with a laser sizer<<SYMPATEC>> of the <<Helos>> type) which is the more filled film (backpart of the composite film support) and, on the other hand, of a PETwith a lower filler content (0.025% of the same silica) of which theouter face will receive a coating of a polyvinyl alcohol(PVA)/water-dispersible copolyester (COPO).

The polyester film has a thickness of 12 micrometres in the differentexamples or comparative tests and an average roughness Rz of 0.45 on theback face. It has a less filled layer (receiving the coating of the PVAand of the water-dispersible copolymer) which has a thickness of 11.2micrometres.

The co-extruded film is first stretched longitudinally with a stretchratio of 3.4; then the same is subjected to a corona treatment whichadjusts it surface tension to 58 mN.m.

The face of the film with the less filled layer is then coated by meansof a gravure coating system using an engraved roller. The speed of thefilm in the coating operation is of 200 m/min. The on-line coating (OLC)is carried out using an aqueous solution of PVA and of COPO having a 10%concentration, carefully prepared and devoid of gel. The PVA used has98-99% vinyl alcohol residues and a viscosity of 5.5 mPa.s (measured at20° C. in a 4% aqueous solution by means of a viscosimeter of theBrookfield LV type. The COPO used is a copolyester of ethylene anddiethylene having an average number molecular weight of about 33000 andincluding terephthalate and isophthalate residues in a ratio of about80/20 in weight, and 14% in weight of sodium 5-oxysulfonyl-isophthalatebased on the total weight of the copolyester.

The coated film is then subjected to a transverse stretching with astretch ratio of 4 and is then subjected to a thermal treatment at atemperature of 225° C. The thickness of the PVA/COPO layer is given foreach example or comparative test.

On the different films dried at 200° C., measurements are made of thepermeability to oxygen at 23° C. with 50% relative humidity (O2P incm³/m²/24 h) (measurements carried out using an <<OXTRAN >> apparatus,type 300H from the firm MODERN CONTROL INC.).

Examples 1 to 3 and Comparative Examples a to c

These tests are carried out with a composite polyester film of which theface receiving the coating of PVA/COPO exhibits the following peakheight distribution, per mm²: 7 peaks of a height> 1 μm, 49 peaks of aheight between 0.4 and 1 μm. The coating is carried out on-line (OLC)with aqueous solutions of PVA/COPO exhibiting a weight ratio PVA/COPO of90/10 (degree of saponification of PVA and overall concentration in thesolution indicated in the table hereafter). O2P was measured on the nonmetallized films (naked films: comparative tests a, b and c).

On one part of these films, an aluminium layer of 0.035 μm was depositedand O2P was measured as previously (metallized films).

In table 1 hereafter, the following abbreviations are used, in additionto the abbreviations defined previously:

<<CT>> stands for <<comparative test>>

<<PVA degree>> stands for <<degree of saponification of PVA.

<<Concsol w/w>> stands for <<total concentration of the aqueous solutionin weight>>

<<Thick>>stands for <<thickness of the PVA/COPO coating>>

<<naked>> stands for non metallized film and <<Al>> stands for filmmetallized with aluminium.

TABLE 1 PVA Concsol Thick, Test degree w/w μm Film O2P Example 1 98% 12%0.1 Al 0.1 Example 2 88% 12% 0.1 Al 0.3 Example 3 80% 12% 0.1 Al 0.2 CTa 98% 12% 0.1 naked 2.0 CT b 88% 12% 0.1 naked 73 CT c 80% 12% 0.1 naked>100

Example 4 and Comparative Test d

The same polyester composite film was used in these tests as in examples1 to 3. In these tests, an on-line coating was carried out using anaqueous solution of PVA/COPO (example 4) or an aqueous solution of PVAonly (comparative test e), with a total concentration of 12% in thesolution. The weight ratio of PVA/COPO was of 90/10 and the PVA had asaponification degree of 98%.

These films were metallized by deposition of an aluminum layer of 0.035μm.

The O2P of the metallized films was measured under the conditionsindicated for the preceding examples.

Each one of the two metallized films was laminated with a polyethylenefilm (of 50 μm) using a polyurethane adhesive and an adhesion test wascarried out.

This test consists in applying on the film a pulling force with anINSTRON 1122 apparatus, at the speed of 100 mm/min. The peeling strengththus measured is expressed in grams per 15 millimetres. A peelingstrength of 150 g 1/5 mm is considered as satisfactory for commerciallysold metallized films. The results of these tests are given in table 2hereafter.

TABLE 2 Peeling strength, Test PVA/COPO O2P g/15 mm Example 4 90/10 0.1220 Comparative example d 100/0  0.1 105

What is claimed is:
 1. A composite metallized film stretched biaxially,based on a polyester and having improved barrier properties to gases,characterised in that it comprises a polyester film base of 5 μm to 50μm, directly coated on one of its two faces with a layer including amixture of a polyvinyl alcohol which has an average degree ofpolymerisation in number equal to or in excess of 350 and awater-dispersible copolyester with sulphonyloxy residues, the layerincluding the polyvinyl alcohol and the water-dispersible copolyesterbeing in turn coated with a metal layer, said composite film exhibitinga permeability to oxygen measured at 23° C. under a relative humidity of50% equal to or lesser than 0.4 cm³/m²/24 h.
 2. A film to claim 1,characterised in that the polyster is selected from polyterephthalatesor from polynaphthalane dicarboxylates of alkylene diols.
 3. A filmaccording to claim 2, characterized in that the polyester is selectedfrom polyterephthalates of ethylene glycol or of 1,4-butanediol or fromcopolyesters including at least 80% in moles of ethylene glycolterephthalate residues.
 4. A film according to claim 1, characterised inthat the average roughness Rz of the film base is lesser than or equalto 0.40 μm on the face of the film carrying the layer of the polyvinylalcohol and of the water-dispersible copolyester and this face exhibits,on the average, not more than 20 peaks having a height equal to or inexcess of 1 μm and not more than 150 peaks having a height comprisedbetween 0.4 and 1 micrometre, per square millimetre, and the thicknessof the layer including the polyvinyl alcohol and the water-solublecopolyester with sulphonyloxy residues is lesser than 0.6 μm.
 5. A filmaccording to claim 1, characterised in that the face of the film basecarrying the layer of the polyvinyl alcohol and of the water-dispersiblecopolyester has, on the average, not more than 20 peaks having a heightequal to or in excess of 1 micrometre and no more than 100 peaks havinga height comprised between 0.4 and 1 micrometre, per square millimetre.6. A film according to claim 1, characterised in that thewater-dispersible copolyesters with sulphonyloxy residues combined withthe polyvinyl alcohol in the coating is selected from copolyestersderived from at least one aromatic dicarboxylic acid and from at leastone aliphatic diol, and carrying a plurality of sulphonyloxy groups ofthe general formula (I): —(—SO₃—)_(n)M  (I) in which n is equal to 1 or2, M is a hydrogen atom, an alkali metal, an earth-alkali metal, acationic ammonium or a cationic quaternary ammonium.
 7. A film accordingto claim 6, characterised in that the aromatic dicarboxylic acid usedfor preparing the water-dispersible copolyester is selected fromterephthalic acid, isophthalic acid, orthophthalic acid, naphthalene1,4-dicarboxylic acid and their mixtures.
 8. A film according to claim7, characterised in that the aromatic dicarboxylic acid is selected frommixtures of terephthalic acid with one or several other aromaticdicarboxylic acids.
 9. A film according to claim 8, characterised inthat the aromatic dicarboxylic acid is selected from mixtures ofterephthalic acid with isophthalic acid.
 10. A film according to claim7, characterised in that the amount of terephthalic acid expressed inmoles is in the range from 20 to 99% of the total number of moles of nonsulphonated diacids and in that the water-dispersible copolyesterincludes sodium 5-oxysulphonyl-isophthalate residues.
 11. A filmaccording to claim 10, characterised in that the amount of terephthalicacid expressed in moles is in the range from 30 to 95% of the totalnumber of moles of non sulphonated diacid.
 12. A film according to claim6, characterised in that the diol entering into the composition of thewater-dispersible copolyesters with sulphonyloxy groups, is selectedfrom ethylene glycol, 1,4-butanediol, 1,5-butanediol, 1,3-propanediol,1,2-propanediol, 2,2-dimethyl-1,3-propanediol (or neopentylglycol),1,5-pentanediol, 1,6-hexanediol, diethylene glycol, triethylene glycol,tetraethylene glycol and cyclohexane dimethanol.
 13. A film according toclaim 12, characterised in that the diol entering into the compositionof the water-dispersible copolyester with sulphonyloxy groups, isselected from ethylene glycol and its oligomers.
 14. A film according toclaim 6, characterised in that in the water-dispersible copolyester, thedicarboxylic acid residues with sulphonyloxy groups amount to 5-30 molesfor 100 moles of the dicarboxylic acid residues.
 15. A film according toclaim 14, characterised in that in the water-dispersible copolyester,the dicarboxylic acid residues with sulphonyloxy groups amount to 8-15moles for 100 moles of the dicarboxylic acid residues.
 16. A filmaccording to claim 1, characterised in that the coating of the polyvinylalcohol and of the water-dispersible copolyester exhibits a weight ratioof the polyvinyl alcohol to the water-dispersible polyester of 99/1 to50/50.
 17. A film according to claim 16, characterised in that thecoating of the polyvinyl alcohol and of the water-dispersiblecopolyester exhibits a weight ratio of the polyvinyl alcohol to thewater-dispersible copolyester comprised between 97/3 and 80/20.
 18. Afilm according to claim 1, characterised in that the coating of thepolyvinyl alcohol and of the water-dispersible copolyester has athickness equal to or lesser than 3 μm.
 19. A film according to claim 1,characterized in that said composite film further includes a layer ofpolyvinyl alcohol disposed on top of the coating of the polyvinylalcohol and of the water-dispersible copolyester, but beneath the metallayer.
 20. A film according to claim 19, characterized in that the metallayer, which is deposited on the surface of the polyvinyl alcohol layeris made of a metal selected from aluminum, copper, chrominum, nickel,and silver.
 21. A film according to claim 20, characterized in that themetal layer, which is deposited on the surface of the polyvinyl alcohollayer is made of aluminum.
 22. A film according to claim 1,characterized in that the metal layer, which is deposited on the surfaceof the coating of the polyvinyl alcohol and of the water-dispersiblepolyester not in contact with the polyester film, is made of a metalselected from aluminum, copper, chromium, nickel and silver.
 23. A filmaccording to claim 1, characterized in that the metal layer, which isdeposited on the surface of the coating of the polyvinyl alcohol and ofthe water-dispersible polyester not in contact with the polyester film,is made of aluminum.
 24. A film according to claim 1, characterised inthat the thickness of the metal layer is in the range from 0.01 μm to0.06 μm.
 25. A film according to claim 1, characterised in that thepolyester base film is comprised of two layers exhibiting differentroughnesses Rz.
 26. A film according to claim 25, characterised in thatthe dissymetric polyester film base is produced with a technique ofco-extrusion of 2 polyesters having different filler contents andpossibly including fillers of differing nature.
 27. A film according toclaim 26, characterised in that the polyester used is the same for thetwo co-extruded layers and in that the layer receiving the polyvinylalcohol layer is the less filled one.
 28. A process for the productionof metallized composite films according to claim 1, characterised inthat the polyester film base is coated on one of its faces with anaqueous solution including a polyvinyl alcohol having, in a 4% aqueoussolution and at 20° C., a viscosity which is equal to or in excess of 4mPa.s and a water-dispersible copolyester, the coating of the polyvinylalcohol and of the water-dispersible polyester being in turn coated, ifdesired, with an aqueous solution of a polyvinyl alcohol, and in thatsaid coated film is subjected to a thermal treatment at a temperatureequal to or in excess of 170° C. and in that a metal layer is depositedon the coating of the polyvinyl alcohol and of the water-solublecopolyester or, on the additional layer of polyvinyl alcohol when such alayer is provided.
 29. A process according to claim 28, characterised inthat the face of the polyester film carrying the coating of thepolyvinyl alcohol and of the water-dispersible copolyester exhibits anaverage roughness Rz lesser than or equal to 0.40 μm and carries, on theaverage, not more than 20 peaks having a height equal to or in excess of1 micrometre, and not more than 150 peaks having a height comprisedbetween 0.4 and 1 micrometre per square millimetre, when the desiredthickness of the coating including the polyvinyl alcohol and thewater-dispersible copolyester is lesser than 0.6 micrometres.
 30. Aprocess according to claim 28, characterised in that the coating of thepolyester film base with an aqueous solution of the polyvinyl alcoholand of the water-dispersible copolyester is carried out on line or as aseparate operation.
 31. A process according to claim 30, characterisedin that the coating is carried out on-line.
 32. A process according toclaim 28, characterised in that the aqueous solution of the polyvinylalcohol and of the water-dispersible copolyester is used at aconcentration in the range from 1% to 20% in weight/weight.
 33. Aprocess according to claim 32, characterised in that the aqueoussolution of the polyvinyl alcohol and of the water-dispersiblecopolyester is used at a concentration in the range from 5% to 15% inweight/weight.
 34. A process according to claim 28, characterised inthat the weight ratio of the polyvinyl alcohol to the water-dispersiblecopolyester in the aqueous solution is between 99/1 and 50/50.
 35. Aprocess according to claim 34, characterised in that the weight ratio ofthe polyvinyl alcohol to the water dispersible copolyester in theaqueous solution is between 97/3 and 80/20.
 36. A process according toclaim 28, characterised in that the temperature at which the coated filmis treated, is between 180° C. and 240° C.
 37. A process according toclaim 36, characterised in that the temperature at which the coated filmis treated, is between 200° C. and 230° C.